In a conventional papermaking operation, wood chips are subjected to severe conditions of acidity or alkalinity at high temperatures in a pulping operation in order to dissolve the more soluble lignin, but not the less soluble cellulosic fibers. After pulping, the cellulosic pulp is subjected to a cleaning operation to remove suspended solids or particulate material. In a typical cleaning operation, debris and heavy material, such as stones, metal, glass, and the like, are initially removed from the pulp by a centrifugal cleaner and the pulp is then subject to coarse screening followed by secondary finer screening to remove large and small sized contaminants. Following this, the pulp is subjected to a forward cleaning operation to remove sand, small fiber bundles, ink and the like, followed by reverse cleaning to remove lighter materials, such as wax, latex, hot melt, and other materials. Following the cleaning the pulp undergoes a washing operation which acts to remove dissolved organic and soluble inorganic material present in the pulp mass. It is necessary to remove dissolved solids because the dissolved solids interfere with bleaching and with the papermaking operation. Further, dissolved solids are a source of increased biological oxygen demand, chemical oxygen demand and color when the dissolved solids are discharged into an effluent system.
After washing, the pulp undergoes stock preparation and the refined and diluted pulp is then utilized in the papermaking machine to produce a paper sheet in the conventional manner.
A common method of washing pulp is a countercurrent system in which fresh wash water is added to the pulp in the final stage of washing and the wash water is then recycled in an upstream direction toward the first stage of the washing operation. With the countercurrent system, the dissolved solids concentration of the wash water increases as it moves from the final stage to the first stage, while the concentration of dissolved solids in the pulp decreases as it travels from the first stage to the final stage.
United States patent application Ser. No. 08/697,271, filed Aug. 21, 1996, now U.S. Pat. No. 5,753,074 describes an improved pulp washing process in which a thin mat of pulp slurry is positioned between a pair of porous endless fabrics or belts to provide a composite structure. The composite structure is then moved upwardly through a washing zone over a series of vertically spaced rolls in a generally sinuous path of travel, such that a first of the belts is in direct contact with one roll, and the second belt is in direct contact with an adjacent roll. As the pulp mat travels through the washing zone, wash water is directed against the mat at the upper end of the washing zone and is cascaded downwardly through the washing zone by a series of baffles. As the pulp mat travels between adjacent rolls, the mat is impregnated with wash water and as the composite structure travels around the rolls, the tension in the belts increases, creating a dewatering action, so that the pulp mat is alternately showered with water and then dewatered by the belt tension.
Thus, the washing process as described in the aforementioned patent removes dissolved solids, as well as suspended fillers, while utilizing a minimum quantity of water, as compared with conventional countercurrent pulp washing operations.